Friday Feature: Frustrating neutrophils

Neutrophils have a special place in the study of cell motility.  It seems that about one in 5 talks on cell motility starts with this classic video of neutrophil crawling, from David Rogers (Vanderbilt) circa 1950 (a YouTube version set to music, just for a little variety):

(credits for original here).

I am not saying this is a bad thing.  It’s a great movie, and a wonderful way to introduce a semi-naive audience to the topic.  And it’s fascinating to see the neutrophil change direction in response to the movement of the bacterium.  How does the neutrophil know where the bacterium has gone?

The signal the neutrophil is responding to is the presence of formylated peptides, which are made by bacteria but not by human cells.  The bacterium is leaving a little trail of breakdown products behind it, rather as we leave behind a trail of dead skin cells and occasional hairs that can be found and identified by a sufficiently well-equipped forensics technician.  But if you think about the diffusion of the signal peptide, there’s a problem: the increase in peptide concentration at the side of the neutrophil that is closest to the bacterium is rather small, so one of the questions that puzzle people is how the neutrophil can “see” this small difference.

The Shah lab is working on this question using a cool microfluidic device developed by Mehmet Toner’s lab at MGH in collaboration with the Mitchison lab.  It consists of many tiny channels that are just the right size for neutrophils to crawl down.  The cells fit so tightly that they block the channel, so it is possible (if you’re careful to keep the pressure on both sides exactly matched) to watch how cells crawl in response to a defined dose of signal peptide (formyl-Met-Leu-Phe).  You can control the concentrations on both sides of the neutrophil independently, so one can ask whether the response to a difference of 10 nM across the cell is different if the concentrations are 0 and 10 nM versus 100 and 110 nM.  The crawling neutrophils look like this:

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Another cool feature is that by changing the size of the channel you can make the neutrophil get stuck.  The normal-size channel, above, is 3 microns high x 8 microns wide.  If you make them a bit tighter, at 1 micron high, the leading edge of the neutrophil can get in, but the rest of the cell body can’t follow; this makes it much easier to study the dynamics of the leading edge.  The Shah lab calls these “frustrated neutrophils”.  Here’s why:

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Oh, the wonderful things you can do with microfluidics.  If you’d like to try these toys for yourself,  Victor Lien would love to hear from you.

Thanks to Harrison Prentice-Mott for the picture and movies!